If you're a stock market bull, beware this chart

With the major stock indexes at record highs, Wall Street is increasingly concerned about the loss of a powerful buying source that may foretell the end of the bull run. Multiple firms sent clients research this week worried about the deterioration of corporate stock buybacks in 2016.

Stock buyback authorizations

Source: Birinyi Associates

Year-to-date corporate stock buyback authorizations are down 29 percent year over year, according to Birinyi Associates. July was even worse with buyback authorizations down 36 percent year over year, the firm told clients Wednesday.